1. Field of Invention
This invention relates to automatic test systems for digital electronic converters and particularly to statistical methods and related electronic apparatus for testing digital-to-analog converters.
Converters between the digital and analog signal domains are employed to interface digital electronic circuitry and analog devices. Accuracy of conversion, gain and repeatability in the process of conversion are matters of concern. One method of testing a digital-to-analog converter is to apply a digital signal to its input to obtain an analog output signal, and then to apply the analog output signal to the input of an analog-to-digital converter to recover a digital signal and then to compare the digital output signal with a standard such as the digital input signal or to process the output signal to determine its statistical characteristics. The characteristics of the output signal in terms of a statistical description provide an indication of the accuracy of the digital-to-analog converter.
2. Description of the Prior Art
State of the art testing techniques for digital converters are generally limited to tesing, in the case of digital-to-analog converters, a few states such as the simple binary weights and full-scale amplitude. Substantially all tests are static in nature and are incapable of providing a complete statistical evaluation of a converter.
Recently dynamic testing techniques have been suggested for limited purposes. An example is a method proposed by L. F. Pau and presented in a paper entitled "Fast Testing and Trimming of A/D and D/A Converters in Automatic Test Systems" at the 1978 IEEE Autotest Conference. Pau suggested analyzing the error signal with Walsh functions. The error signal is obtained by comparing to a reference device. Other known evaluation methods also presupposed the use of an ideal reference device, such as an analog-to-digital converter having a linear transfer characteristic, the output response of which is employed with the output response of a device under test to generate an error signal from which an evaluation of performance is made. A device built according to the technique suggested by Pau is therefore limited by the accuracy of the reference device.
One of the concerns of prior art converter testing methods has been an inability to distinguish between signal-induced biases or errors and biases or errors inherent in the testing devices. Accordingly, much effort has been devoted to linearization of the excitation signals and of the transfer characteristics of the measuring devices, such as the analog-to-digital converter used to detect the output of the digital-to-analog converter under test. The present invention represents a departure from this effort to idealize the testing equipment.
3. Cross-Reference to Related Application
In the copending parent patent application entitled "Method and Apparatus for Digital Converter Testing," a method and apparatus are disclosed for dynamically testing the overall performance characteristics of both digital-to-analog converters and analog-to-digital converters. The method comprises dynamically exercising an analog or digital converter with, respectively, analog or digital signal patterns which can be characterized as the sum of a set of mutually orthogonal two-state functions of defined amplitudes, the sum having substantially uniform amplitude distribution over the allowable states of the amplitude range in order to exercise substantially all allowable converter states of concern at random times. Thereafter, the response of the converter under test is examined for a number of basic performance parameters, including total distortion, linearity and optimum gain. The method described therein yields a relatively complete statistical description of the performance characteristics. The disclosure of that application is incorporated herein by reference and made a part hereof.